1. Field of the Invention
This invention relates to dispersing polymers for phthalocyanine pigments. In particular, this invention relates to dispersing polymers that provide highly dispersed and stable methyl ethyl ketone dispersions of phthalocyanine pigments for use in electrophotographic applications.
The present invention also relates to an electrophotographic organic photoconductor using phthalocyanine pigments dispersed in a dispersing polymer to provide charge-transport and charge-generating characteristics in a high performance organic photoconductor.
2. Background of the Invention
The phthalocyanine class of pigments has proven to be very useful colorants in a wide variety of applications. Because of their color purity and transparency, the phthalocyanine pigments are well known for their excellent color matching capabilities in applications, such as, color proofing, printing inks, colored films, liquid electrostatic toners, etc.
In addition, phthalocyanine pigments dispersed in a polymeric binder system are useful in electrophotography as charge generating/transporting materials in organic photoconductors. Electrophotography forms the technology base for a variety of well known imaging processes, including photocopying and laser printing. The process involves placing a uniform electrostatic charge on a photoconductor element, imagewise exposing the photoconductor element to light thereby dissipating the charge in the exposed areas to form an electrostatic latent image, developing the resulting electrostatic latent image with a toner, and transferring the toned image from the photoconductor element to a final substrate, such as paper, either by direct transfer or via an intermediate transfer material.
Photoconductor elements based on organic materials have received significant emphasis due to their flexibility, the dark resistivity and radiation sensitivity of organic materials, and lower cost of materials and manufacturability. See for example, Borsenberger, P. M., et al, Photoreceptors: Organic Photoconductors, Handbook of Imaging Materials, Ed. A. S. Diamond, Marcel Dekker, Inc., New York, N.Y., Chap. 9, 379 (1991); and Borsenberger, P. M., et al, Photoreceptors, Organic Photoreceptors for Imaging Systems, Marcel Dekker, Inc., New York, N.Y., Chap. 11, 301 (1993). In particular, both metal contained and metal-free phthalocyanine pigments have been the focus of extensive research as charge generating and charge transporting materials in both negatively and positively charged organic photoconductors. X-metal-free phthalocyanine pigments have been used both for their charge generating and charge transporting functions in single layer constructions, and for their charge generating function in dual layer constructions.
Phthalocyanine pigments are one of more difficult classes of pigments to form highly dispersed and stable liquid dispersions, especially in methyl ethyl ketone (MEK) solvent. The use of MEK is desirable since there is a preponderance of manufacturing experience in both dispersion and coating processes for a wide variety of product applications. In addition, little residual solvent is left behind in coatings upon drying of MEK coating solutions because of its volatility.
The quality of the phthalocyanine dispersion has a direct relationship upon the performance of the organic photoconductor. Typically organic photoconductors use phthalocyanine pigments dispersed in polyvinylacetal binders. Solvents such as tetrahydrofuran, methylene chloride, or one of the cellosolve based solvents are primarily used in these applications to achieve efficient charge transport properties.
Many attempts have been made to improve both the quality and the stability of phthalocyanine dispersions. In U.S. Pat. No. 5,364,727, a single layer photoconductor is described containing a distribution of phthalocyanine pigment and arylamine sensitizer in a polymeric binder having polar and non-polar functional moieties. The polar function of the polymer, comprising esters, carbonyl and amide groups, is believed to stabilize the phthalocyanine pigment dispersion. The non-polar function of the polymer, comprising alkanes and alkenes, is believed to provide absorption of the hydrocarbon solvent of the liquid toner. The only solvent disclosed is a chlorinated solvent, specifically dichloromethane.
Incorporation of an ammonium component into a pigment treatment resin is described in U.S. Pat. No. 4,618, 554. The treatment resin comprises an aqueous soluble acrylic resin with a pendant alkyl ammonium group attached. A pigmented photoreceptor solution is produced using a two step process. The pigment is first treated by mixing the acrylic resin with the pigment under harsh acid conditions. The material is then isolated and neutralized before dispersing it into a solvent based photoreceptor coating solution.
U.S. Pat. No. 5,028,506 describes the addition of a low molecular weight ammonium salt to a charge-generating (pigment) dispersion to provide an electrophotographic photoreceptor with improved repetitive characteristics without lower the sensitivity. The ammonium salt is a post additive to the pigment dispersion and not a dispersing aide for improving dispersion quality.
U.S. Pat. No. 5,087,540 describes a phthalocyanine/poly(vinylbutyral) dispersion for organic photoconductor applications having a "molecularly dissolved" state, which is necessary for an effective photoconductor performance. In addition, methyl ethyl ketone is identified as an "undesirable solvent" for metal-free phthalocyanine pigment dispersions.
The solvents disclosed in the art which give acceptable phthalocyanine dispersions present several toxicological and environmental issues. The chlorinated solvents are well known to cause environmental problems. In addition, the chlorinated solvents are suspected carcinogens and have been banned from use in some jurisdictions. Cellosolve solvents are suspected as carcinogens and teratogens. MEK has better toxicological and environmental properties compared to the chlorinated and cellosolve solvents. Tetrahydrofuran (THF), if not properly treated to prevent the formation of peroxides, can cause an explosion. Even when anti-oxidants are used with TIff their effect is only temporary; thus requiring special handling during storage and solvent recovery operations. Unlike THF, MEK does not form peroxides easily in the presence of oxygen, light or heat.
Polymers with attached ammonium groups or ammonium compounds have also been used in the production of dry electrostatic toners. In U.S. Pat. Nos. 4,299,898; and 4,224,396; dry electrostatic toners are described where a pigment is dispersed in a resin comprising an acrylate polymer with a quaternary ammonium salt attached to the polymer. In U.S. Pat. Nos. 5,215,848; 4,221,856; and U.S. Pat. Re. No. 32,883; dry toners are described where a quaternary ammonium compound is added to the dispersion. However, the primary function of the quaternary ammonium groups in each of the above applications is to impart a stable positive charge on the toner. There is no indication that the use of quaternary ammonium salts may be useful in a liquid dispersion or for improved performance of a photoconductive layer in a photoconductor.
U.S. Pat. No. 5,139,892 describes a magnetic recording media which uses a vinyl chloride copolymer having pendant quaternary ammonium groups to disperse magnetic particles. The disclosure does not contemplate the use of such polymers as a phthalocyanine pigment dispersant.
There is a need for a dispersing polymer which can form a highly dispersed and stable phthalocyanine pigment dispersion in a more suitable solvent.